Yam (vegetable)

• 1. Introduction

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  Yam is the common name for certain plants in the genus Dioscorea, and among the Igbo people of south eastern Nigeria it comes with a rule. No one eats the newly harvested tuber until the community has marked the feast called Iri ji ọhụrụ. A starchy root that can hold a whole people back from its first bite of the season is plainly more than a side dish. So what exactly is a yam? Why does the same word name entirely different plants in different countries? How did one family of vines become a crop on three separate continents? And what does it take to dig, keep and cook a tuber that fights back at every stage?

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• 2. A Name That Travels

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  The English word yam appears to trace back to Portuguese inhame and Canarian Spanish ñame. Both of those came from Fula, one of the West African languages, during trade. The twist is that in Portuguese, inhame commonly refers to taro, Colocasia esculenta, a plant from a different genus. Names for true yams across the world's regions are many, and the main derivations borrow from verbs meaning "to eat".

  The confusion spreads from there. In the United States, sweet potatoes with orange flesh are often called yams. In New Zealand, the word usually means oca, Oxalis tuberosa. In Malaysia and Singapore, it means taro. In Australia, the tubers of Microseris walteri, called the yam daisy, were a staple food of Aboriginal Australians in some regions.

  Across Africa, South and Southeast Asia and the tropical Pacific islands, another plant carries the label too. Amorphophallus paeoniifolius is grown there and goes by the name elephant foot yam, though it is no relative of the true yams.

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• 3. Vines and Voyages

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  Some 870 species of Dioscorea are known, in the family Dioscoreaceae. They are monocots, related to lilies and grasses, and they grow as vigorous perennial vines rising from a tuber. A vine can reach 15 metres in length, while the tuber may push as deep as 1.5 metres into the soil. The plant spreads by seed.

  The tuber itself has a rough skin that is hard to peel but softens readily in cooking. Skins range from dark brown to light pink. Inside, mature tubers show flesh from white or yellow to purple or pink.

  Three species were domesticated independently, one on each of three continents. D. rotundata was brought into cultivation in Africa, D. alata in Asia and D. trifida in South America.

  The Asian species then went to sea. D. alata and the lesser yam, D. esculenta, were staple crops for the seafaring Austronesian cultures. Sailors carried them as canoe plants from Island Southeast Asia as far as Madagascar and Polynesia.

  Historical records of yams in West Africa, and of African yams in Europe, reach back to the 16th century. Portuguese and Spanish carriers took the crop across the Atlantic, with its route running along the borders of Brazil and Guyana before it spread through the Caribbean.

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• 4. The Cultivated Cast

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  More than 200 varieties are cultivated between the yellow yam, D. cayennensis, and the white yam, its subspecies rotundata. Both are native to Africa, and they are the most important cultivated yams. Taxonomists once ranked the white yam as its own species, D. rotundata, but most now treat it as a subspecies. White yam tubers are roughly cylindrical, with smooth brown skin and firm white flesh. Yellow yam owes its colour to carotenoids. The vines run 10 to 12 metres, and tubers usually weigh 2.5 to 5 kilograms, though some reach 25. Harvest comes after 7 to 12 months. The Kokoro variety is prized for dried yam chips, and most African harvests are pounded into the paste called iyan.

  D. alata has many names, among them purple yam, greater yam, winged yam and water yam. It was first cultivated in Southeast Asia, and it has the widest distribution of any cultivated yam, reaching Asia, the Pacific islands, Africa and the West Indies. Even in Africa, only white yam is more popular. Its flesh is white and watery. The Okinawan purple "yam" sometimes confused with it is actually a sweet potato.

  The Chinese yam, D. polystachya, is native to China and grows vines of about 3 metres. It tolerates frost, so it thrives in cooler conditions than its tropical cousins. Europe received it in the 19th century, when the potato crop was falling to disease. France still grows it for the Asian food market. After about 6 months, some tubers are eaten fresh, while others go into noodles and traditional medicines.

  The lesser yam, D. esculenta, was among the first yams ever cultivated. It is the third most commonly grown species in Southeast Asia and rare elsewhere. Its small tubers make mechanical cultivation possible, which could one day widen its appeal.

  In the Guyana region of South America, the cush-cush yam, D. trifida, is the most important cultivated yam of the New World. It evolved in tropical rainforest, so its growth cycle follows the seasons less closely than other yams do.

  Two older African tuber crops, the Hausa potato, Plectranthus rotundifolius, and the Livingstone potato, P. esculentus, have been largely displaced by the arrival of cassava.

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• 5. The Wild Side

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  The bitter yam, D. dumetorum, is popular in parts of West Africa partly because it takes less labour to grow than other yams. Its wild forms are very toxic. They have been mixed with bait to poison animals, and it is said they have served criminal purposes too. Unlike cassava, most mature cultivated yams carry no toxic compounds. Even so, bitter compounds gather in the immature tissue of white and yellow yams.

  Bitter yams contain toxins such as dihydrodioscorine. They are eaten only in desperation, during local food scarcity. Cooks detoxify them by soaking them in salted water, in cold or hot fresh water, or in a stream. The bitter compounds are water-soluble alkaloids, and swallowing them produces severe and distressing symptoms. The worst cases of poisoning can be fatal.

  The air potato, D. bulbifera, grows in both Africa and Asia as a vine of 6 metres or more. Its prized harvest is the bulbils on the leaf bases, which weigh 0.5 to 2.0 kilograms each. Some varieties can be eaten raw, while others need soaking or boiling first. In Asia, bitter cultivars are treated by water extraction, fermentation and roasting of the grated tuber. The bitter compounds include diosbulbin and possibly saponins such as diosgenin. In Indonesia, an extract of air potato goes into arrow poison.

  Home gardeners like the plant because it yields after only four months and keeps producing for as long as two years. In 1905 it reached Florida, where it became invasive across much of the state. It crowds out native plants, and cutting or burning does not end it. New vines sprout from the tubers and from fallen bulbils.

  Some wild yams are simply food. In southern Zambia, D. hirtiflora subsp. pedicellata, called lusala, busala or lwidi, grows in open forest. It adds to the diet of almost all households from March to September, and it brings income to over half of rural households. Researchers have succeeded in propagating it, which should ease pressure on wild stands.

  Other wild tubers have a quite different market. Species such as D. nipponica, along with a variant of Kokoro yam, yield diosgenin, a steroid compound. Laboratories convert it into cortisone, pregnenolone and progesterone. Such preparations went into early combined oral contraceptive pills.

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• 6. Field and Harvest

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  The rains set the calendar. A yam crop begins with whole seed tubers or pieces of tuber planted into mounds or ridges as the rainy season opens. Growth follows the wet season, and dormancy follows the dry one. For the best yields, the plants need a humid tropical climate with more than 1500 millimetres of rain a year, spread evenly through the growing months.

  The seed itself is a burden. Seed yams are perishable and bulky to haul, so farmers who do not buy new ones set aside up to 30 percent of their harvest for the next planting. Small-scale farmers in West and Central Africa often grow yams among cereals and vegetables. White, yellow and water yams typically make one large tuber a year, usually 5 to 10 kilograms. Insects, fungal and viral diseases and nematodes all press on the crop. One fungus, Colletotrichum gloeosporioides, is found in growing regions worldwide and afflicts many Dioscorea species.

  Digging comes next, and it is punishing. In West Africa, workers dig by hand with sticks, spades or diggers. They prefer wooden tools because metal is more likely to wound the fragile tubers, though wood wears out fast. Harvesters stand, bend, squat and sometimes sit in the dirt, depending on the mound and how deep the tuber lies. A nicked tuber stores badly and spoils quickly. In forest, tubers tangle with tree roots and must be freed, which often damages them. The bulbils of air potato are simply plucked from the vine.

  Mechanization could lower costs and raise yields, but the way yams are grown stands in the way. Mixed cropping would probably have to change, and harvesting equipment would need redesign to suit the shape and texture of the tuber.

  The effort still pays off. In 2020, the world produced 75 million tonnes of yams, and Nigeria alone supplied 67 percent of that total. About 95 percent of yam crops are grown in Africa. Demand is high enough in certain African subregions to make the work profitable. Far away in Colombia, yams are a key food on the Caribbean coast. In 2010, the country ranked among the 12 largest producers and first in yield per hectare. Studies have shown possible uses in pharmaceuticals and bioplastics, yet Colombia has no recorded use beyond food.

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• 7. Storage and Nutrition

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  A harvested yam is still alive. Tubers keep respiring in storage, and that process oxidizes their starch into water, carbon dioxide and heat, steadily shrinking the dry matter. Even so, properly stored yam is considered the least perishable of the major roots and tubers. Good storage depends on picking sound tubers, curing them, ideally with a fungicide treatment, and providing ventilation to carry off the heat. Keepers must also inspect often, pull out rotting tubers and sprouts, and shield the pile from sun and rain.

  Cold is not the answer. Below 12 degrees Celsius, yams suffer chilling injury, with internal tissue breaking down, water loss rising and decay setting in. The damage often shows only once the tubers return to ordinary temperatures. The ideal range is 14 to 16 degrees with controlled humidity, after curing. Most yam-growing countries are too poor to afford such systems. Sprouting speeds respiration sharply and drains the tuber's food value faster.

  Variety matters. Yams from arid climates stay dormant longer and keep better than those bred for humid tropical lands. Yellow yam and cush-cush yam rest for much shorter periods than water yam, white yam or lesser yam. In Africa, bacteria, insects, nematodes and mammals cause very high storage losses. Still, some varieties last up to six months without refrigeration, which helps households through the yearly food shortage at the start of the wet season.

  Raw yam has modest nutrient density. Only potassium, vitamin B6, manganese, thiamin, dietary fibre and vitamin C reach 10 percent or more of the Daily Value. Yet its potassium level is the highest among the 10 major staple foods of the world. A 100-gram serving supplies 494 kilojoules. The glycemic index runs about 54 percent of glucose for a 150-gram serving, lower than for potato products.

  Protein is the weak point. Yam holds about 2 percent protein on a fresh-weight basis, and its amino acid balance is poor. Together with cassava, it supplies from 5.9 percent of protein intake in East and South Africa to about 15.9 percent in humid West Africa. Experts urge adding protein-rich foods to yam-heavy diets so children grow well.

  All the same, yam gives more than 200 calories per person per day to over 150 million people in West Africa. It suits poor farms with scarce resources, and it is available year-round, unlike unreliable seasonal crops.

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• 8. Plates and Festivals

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  In Ghana, the Akan boil yam and mash it with palm oil into eto, served with eggs. Boiled yam can also be pounded in a mortar into iyan, a thick paste eaten with egusi or palm nut soup. In Nigeria, sun-dried yam pieces turn dark brown and are milled into a powder called elubo. Stirred into boiling water, it becomes the pudding-like amala, eaten with local soups. African yams must be cooked, since natural substances in them can cause illness when raw.

  In Pernambuco state in Brazil, sliced boiled yam appears at breakfast with cheese spread or molasses. In the Philippines, purple ube is sweetened into ube halaya, and it also goes into halo-halo and ice cream. Vietnamese cooks mash yam and cook it very thoroughly for soups such as canh khoai mỡ. Peasants in the Red River and Mekong delta regions used yam root to dye cotton clothes until the mid-20th century, and people in the Sapa region still do. In Indonesia, mashed purple yam is mixed with coconut milk and sugar for desserts, while white-fleshed yams are cubed, cooked, lightly fermented and eaten as afternoon snacks.

  Japan breaks the cooking rule. The mountain yam, nagaimo, is eaten raw and grated after a brief soak in vinegar water, which neutralizes irritant oxalate crystals in the skin. It comes in forms called ichōimo and yamatoimo. Jinenjo goes into soba noodles. In Okinawa, purple yam is fried as tempura, grilled, boiled, or baked into ice cream and cake with a striking purple colour.

  In central India, cooks slice yam finely, spice it and deep fry it. In the south it accompanies rice and curry. Digging and selling taradi, the regional name for several species, is a major source of income around Palampur. Nepal eats Dioscorea root at the midwinter festival of Māgh Sankrānti. In Fiji, yam is boiled, roasted in a lovo or steamed with fish or meat, though it costs more than cassava or taro because it is hard to farm and produced in small volume. Jamaica honoured yams in ceremonies, as many West African traditions do. In Papua New Guinea, where yams are called kaukau, planting and harvesting come with complex rituals and taboos. In the Loyalty Islands of New Caledonia, stories of how yams first arrived are told in Pene Nengone, with one version from Maré. In the West, African grocers sell yam powder, which lumps easily when stirred into boiling water.

  The grandest celebration belongs to West Africa. The Igbo have long treated yam as their main staple, central to both farming and culture. In Nigeria and Ghana, a yam festival usually falls in early August, at the end of the rainy season. Offerings go first to gods and ancestors, and only then are yams shared with the villagers. The New Yam Festival honours the main crop of the Igbos, Idomas and Tivs. In Owukpa in Idoma west it is called Orureshi, and in Igbo land it is Ima-Ji, Iri-Ji or Iwa Ji.

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